The present invention relates to electroexplosive devices (EEDs) such as detonators, blasting caps and squibs. In particular, this invention relates to a method and device for desensitizing EEDs to electromagnetic radiation and electrostatic charges with the added ability to desensitize the device to essentially dc currents. This problem heretofore was considered intractable as the essentially dc currents induced by equipment arcing were similar to the system firing currents used to initiate the EED.
This is an improvement on Monolithic RF/EMI Desensitized Electroexplosive Device, the subject of a patent application, Ser. No. 280,049 filed on 12/05/88 by inventor, Thomas A. Baginski.
A variety of propulsion systems and ordnance depend upon an electrical signal to initiate combustion. This signal is typically a dc current. The current flows through a conductor (typically a bridgewire supported between two posts) which causes a rapid temperature rise via ohmic heating. Once the conductor reaches a sufficiently high temperature it ignites nearby material. The ignited material is then used to initiate combustion of secondary material. The device which consists of the conductor and primary combustionable material is referred to as an electroexplosive device (typically referred to as an EED or squib).
Over the past four decades the electromagnetic environment of an electroexplosive device has changed dramatically. The operation of high-power radar and communication equipment has introduced high-intensity electromagnetic fields to the environment. This problem is especially acute onboard Naval carriers with their multitude of high intensity electromagnetic sources.
The fields can be coupled into an electroexplosive device. The methods of coupling are direct radio frequency (RF) radiation (e.g., the EED acts as the load of a receiving antenna) and arcing associated with weapons procedures such as the attachment of an umbilical cable. These two events will be referred to as electromagnetic interference (EMI).
In an arc, the signal which is coupled into the EED has a wide range of frequency components including very low frequency and dc currents. Prior art thinking was generally that the dc and essentially dc signals, i.e., very low RF frequencies, were intractable problems as the firing circuits employed dc or a very low frequency, e.g., 400 hertz signals, to initiate the EED. Nearly all of the energy of the dc or essentially dc signal is almost instantaneously coupled into conventional bridgewire. Energy per unit time is power, which in our case can be extremely high because of the short time involved in an arcing event. Since power is high, a conventional bridgewire will heat and ignite or dud.
Various methods have been used to alleviate the problem of misfiring caused by electromagnetic radiation. Prior art systems have included inductive and capacitive components that form a balanced bridge or a tank to shunt unwanted signals from the bridgewire. One such protection device is disclosed in Parker et al., U.S. Pat. No. 3,181,464 issued May 4, 1965, which employs special conductors. Parker is used with EEDs having an exploding bridgewire. Other prior art devices add discrete components such as capacitors and inductors to form RF filters or otherwise electronically shunt unwanted signals away from the bridgewire. For example, Jones, U.S. Pat. No. 4,304,184 uses one or more inductors and ferrite beads to oppose and/or absorb unwanted current flow. Proctor et al., U.S. Pat. No. 4,378,738 passes the leads through ferrite chokes. Heretofore however, no device of monolithic construction, small enough to be used in small calibre ammunition, could deal with the dc and essentially dc components associated with an arcing event.
Also, prior art devices are often unsuitable for commercial production because of high manufacturing costs. The constant downsizing of ordnance requires a greater degree of miniaturization at the same time that greater efficiency and the ability to handle higher induced currents is required. As a result, a new design for a miniaturized highly effective EEd that is desensitized to both RFI and EMI is required; one that can endure essentially dc currents when an arcing event occurs.